Gradient along object hair particles (like grass)?
A couple problems arise from using the Object coordinates. I would use Vertex Colors:

In Weight Paint mode, use the Weight Gradient tool under Weight Tools in the T sidebar to create a weight gradient along your grass blade. Use the weight painting tools to adjust as needed:



Next, enable the Tissue add-on in the Add-ons panel of the user preferences:



Now, in Vertex Paint mode, use the Vertex Color from Vertex Group tool in the Tissue Tools panel of the T sidebar to convert the weight into a vertex group:



Last, use an Attribute node to get the gradient from the vertex color:



Result as a particle: